1. Field of the Invention
The present invention relates to thermal print heads being used in thermal recording devices such as facsimile machines, video-printers, and engraving printing.
2. Description of the Related Art
Thermal heads, being advantageous in various points such as low noise, low maintenance cost and low running cost, are much used in thermal recording devices of various kinds of office automation devices such as facsimile machines, video-printers or plate making machines.
In general, a thermal head comprises a base, a printed circuit board, and a semiconductor element for driving. The base is provided with heat-generating resistors formed on a heat sink member made of metals of good thermal conduction such as for instance as aluminum. The printed circuit board supplies electricity to the heat-generating resistors. The semiconductor element for driving is formed on either one of the base or the printed circuit board. Here, the base that is provided with the heat-generating resistors and the printed circuit board is electrically connected through the semiconductor element for driving.
On the base, with a main object of improving surface smoothness and heat accumulation property, a graze glass layer is arbitrarily formed. On the graze glass layer, after forming a heat-generating resistor layer and conductor layer such as aluminum, a process of photoengraving is implemented to form a plurality of heat-generating resistors and electrode patterns.
The semiconductor elements for driving are frequently formed on the bases in plate-making machines. On the other hand, in the video-printers, in many cases, the semiconductor elements are formed on the printed circuit boards.
A sealing-member seals to protect an electrical connection between the semiconductor element for driving, and the base provided with the heat-generating resistors and the printed circuit board. The sealing-member consists of resin materials based on such as thermosetting organic resins.
The thermosetting organic resin based materials, if not affecting adversely on the electrical connection, are not particularly restricted. Epoxy-resin based materials, being mechanically strong enough at room temperature after curing, high in adhesion to the base, and deficient of tacky properties on the surface thereof, are suitable. On the other hand, silicone resin based materials that are relatively flexible after curing can be used.
When the silicone based resin materials that are flexible after curing are used as sealing-material, by a metal or resin cover, connection lines and electrical connection and at least part of the base, printed circuit board, semiconductor element for driving, or sealing-member are necessary to be protected from external force. This cover electrically, mechanically and environmentally protects the semiconductor element or the like. This cover is not necessary when sealing-member is such as epoxy-resin or the like that is sufficiently strong when cured.
FIGS. 6 to 9 show cross sections of an existing and general thermal head.
As shown in FIGS. 6 to 9, on a heat sink member 3, a ceramic substrate 1 that is the base and a PCB (Printed circuit board) 2 that is the printed circuit board are adhered through an adhesive layer 4. The heat sink member 3 is made of metal such as aluminum of good thermal conduction. On the ceramic substrate 1, heat-generating resistors 9 and electrode patterns connected to the heat-generating resistors 9 are formed. The PCB 2 supplies an electric current for heating the heat-generating resistors 9, being constituted of for instance epoxy-based material.
The semiconductor element for driving (driving IC) 5 is disposed on either one of the ceramic substrate 1 or the PCB 2. The electrode patterns on the ceramic substrate 1 and the PCB 2 are electrically connected through the driving IC 5 by bonding wires 6 or the like. Further, the driving IC 5 and the electrical connection are sealed and protected by thermosetting organic resin materials 7A or 7B.
In FIGS. 6 and 7, epoxy based resin 7A is used as a sealing-member. In FIG. 6, the driving IC 5 is disposed on the PCB 2 side, and in FIG. 7, the driving IC 5 is formed on the ceramic substrate 1 side that is the base thereon the heat-generating resistors 9 are formed.
In FIGS. 8 and 9, silicone based resin 7B is used as a sealing-member. The silicone based resin 7B being relatively flexible even after curing, the metal cover 8 is fixed by a screw (not shown in the figure) to fix and protect from external force. In FIG. 8, the driving IC 5 is formed on the PCB 2 side, and in FIG. 9, the driving IC 5 is formed on the ceramic substrate 1 side that is the base thereon the heat-generating resistors 9 are formed.
As mentioned above, in the general thermal print head, the ceramic substrate 1 having heat-generating resistors 9 and the printed circuit board 2 for heating the heat-generating resistors 9 are electrically connected through the driving IC 5 by bonding wires or the like. The thermosetting organic resin material 7A or 7B protects the driving IC 5 and the electrical connection.
The glass epoxy based PCB 2 has a thermal expansion coefficient larger than that of the ceramic substrate 1. Accordingly, when the epoxy resin based material 7A is coated covering both the ceramic substrate 1 and the printed circuit board 2 followed by curing, at the electrical connection after curing, the PCB 2 is connected and fixed in a more expanded state than the ceramic substrate 1.
Upon returning from this state to room temperature, due to difference of contractions of the ceramic substrate 1 and PCB 2, both bends in a plane of the substrate to result in bending of the heat-generating resistors 9 formed on the ceramic substrate 1.
When the heat-generating resistors 9 bend, images are printed bent in their shapes. Further, between end portions and a center portion of a main scanning direction of the heat-generating resistors 9, there occurs a difference in positional relation with a platen roller 10. Accordingly, the images are printed less uniformly in shade. Thus, due to the bending of the heat-generating resistors 9, image quality deteriorates.
In addition, due to the bending of the ceramic substrate 1 and PCB 2 in a plane of the substrate, stress concentrates between the ceramic substrate 1 and PCB 2 and the adhesive layer 4, and the electrical connection. Due to the concentration of the stress, peeling of the ceramic substrate 1 and PCB 2 from the adhesive layer 4, snapping of the bonding wire 6, and peeling or crack of the epoxy resin based material 7A may occur.
On the other hand, when the silicone resin based material 7B is used to seal and protect the electrical connection, the silicone resin based material being relatively flexible after curing, the bending due to the difference of contractions of both substrates is alleviated. Thereby, the bending of the heat-generating resistors 9 is reduced. Accordingly, the deterioration of the image quality and an influence due to the stress concentration can be alleviated.
By contrast, the silicone resin based material 7B is not large in mechanical strength. Accordingly, as mentioned above, to protect the electrical connection from the external impact or the like, it is necessary to provide a cover 8. Provision of the cover 8 causes an influence on the size of the ceramic substrate 1, the position of the heat-generating resistors 9 and the diameter of the platen roller 10. That is, the cover 8 physically hinders from moving the platen roller 10 toward the PCB 2. As a result of this, the ceramic substrate 2 that is expensive can not be downsized and cost reduction becomes difficult.
The present invention is carried out to solve the aforementioned problems and has the following objects.
(1) Expansion due to difference of thermal expansion coefficients of a ceramic substrate and a glass epoxy based printed circuit board is absorbed to provide a thermal print head of excellent printing quality.
(2) To provide a thermal print head of high reliability, further of smaller size.
In order to achieve the aforementioned objects, in the present invention, a thermal print head is constituted in the following manner.
(1) A thermal print head for recording on a recording medium in one mode of the present invention comprises a base provided with heat-generating resistors, a printed circuit board disposed in close proximity to the base, a semiconductor element for driving, and a sealing-member. The semiconductor element for driving is mounted on the base or the printed circuit board and electrically connected to the base and the printed circuit board. The sealing-member seals the semiconductor element for driving and includes a hard seal portion consisting of a hard sealing-material and a soft seal portion consisting of a soft sealing-material more soft than the hard sealing-material. Here, the hard seal portion is disposed on the base, and the soft seal portion is disposed extended on the base and printed circuit board.
The hard seal portion being disposed on the base, this portion does not require a protection due to a cover or the like. The soft seal portion being disposed on the boundary area between the base and the printed circuit board, the bending of the heat-generating resistors can be tempered.
As a result of this, improvement of image quality of the printed images and downsizing of a thermal print head can be accomplished simultaneously.
(2) A thermal print head in one mode of the present invention comprises a base provided with heat-generating resistors, a printed circuit board disposed in close proximity to the base, a semiconductor element for driving, and a sealing-member. The semiconductor element for driving is mounted on the base or the printed circuit board and electrically connected to the base and the printed circuit board. The sealing-member seals the semiconductor element for driving and includes a hard seal portion and soft seal portion. The hard seal portion consists of a hard sealing-material. The soft seal portion consists of a soft sealing-material more soft than the hard sealing-material. Here, the hard seal portion is disposed on the base, the soft seal portion is disposed on the printed circuit board, and the hard seal portion and the soft seal portion are connected.
The soft seal portion being disposed on the printed circuit board, the bending of the heat-generating resistors can be tempered. In addition, the hard seal portion being disposed on the base, this portion does not need protection due to the cover or the like. Still further, the hard seal portion and the soft seal portion being connected, the sealing-member can be improved in strength.
As a result of this, improvement of image quality of the printed images and downsizing of a thermal print head can be accomplished simultaneously.
(3) A thermal print head for recording on a recording medium in another mode of the present invention comprises a base provided with heat-generating resistors, a printed circuit board disposed in close proximity to the base, a semiconductor element for driving, and a sealing-member. The semiconductor element for driving is mounted on the base or the printed circuit board and electrically connected to the base and the printed circuit board. The sealing-member seals the semiconductor element for driving and includes a hard seal portion and soft seal portion. The hard seal portion consists of a hard sealing-material. The soft seal portion consists of a soft sealing-material more soft than the hard sealing-material. Here, the soft seal portion is disposed extended on the base and the printed circuit board, the hard seal portion covering the soft seal portion.
On most surfaces of the base and printed circuit board, the soft sealing-member of high flexibility is disposed. Thereby, the bending of the heat-generating resistors can be tempered. The soft sealing-member being covered by the hard sealing-member, the cover or the like is unnecessary for protection.
As a result of this, improvement of image quality of the printed images and downsizing of a thermal print head can be accomplished simultaneously.